Flask-Security-Too OAuth reauthentication freshness bypass via cross- user OAuth identity acceptance

SECURITY_OAUTH_ENABLE = True
SECURITY_OAUTH_BUILTIN_PROVIDERS = ["github"]
SECURITY_FRESHNESS = timedelta(seconds=1)
SECURITY_FRESHNESS_GRACE_PERIOD = timedelta(seconds=0)
SECURITY_USERNAME_ENABLE = True
SECURITY_CHANGE_USERNAME = True

The OAuth provider was replaced with a localhost mock provider
returning attacker@example.com. This avoids hitting a live third-party
provider while still exercising Flask-Security-Too's real OAuth
verification handler.

Reproduction steps:

1. Log in as victim@example.com.
2. Wait until the session is no longer fresh.
3. Confirm POST /change-username is blocked with 401 and
   reauth_required=true.
4. Start OAuth verification with POST /login/oauth-verify-start/
   github.
5. Complete the callback with an OAuth identity for
   attacker@example.com.
6. Confirm the session is still for victim@example.com, but fs_paa has
   been updated.
7. Retry POST /change-username.
8. The victim user's username is changed successfully.

Observed result:

{
  "pre_bypass_status": 401,
  "pre_bypass_reauth_required": true,
  "attacker_identity": "attacker@example.com",
  "oauth_verify_response_status": 302,
  "post_bypass_change_username_status": 200,
  "final_email": "victim@example.com",
  "final_username": "victimowned1777878574",
  "direct_impact_verified": true
}

Note: CSRF was disabled in the local harness only to keep the test
focused on the reauthentication check. This is not a CSRF bypass
report.

This bypasses Flask-Security-Too's freshness/reauthentication
boundary.

Applications using OAuth verification together with freshness-
protected account operations may allow a stale victim session to be
refreshed using a different user's OAuth account. In my test, this
allowed the victim account's username to be changed through Flask-
Security-Too's built-in /change-username route.

A likely fix is to reject OAuth verification unless the resolved OAuth
user matches current_user before updating session["fs_paa"].